Add dependencies

Declare dependencies in besa.toml. For example, the TOML equivalent of a development-only CMake provider dependency is:

[[dependencies]]
name = "Catch2"
version = "3"
kind = "dev"
provider = "cmake"

A conditional documentation dependency is:

[[dependencies]]
name = "Doxygen"
kind = "dev"
provider = "cmake"
when = { all = ["user-docs"] }

Supported dependency kinds are normal, build, and dev; providers are cmake and pkgconfig. visibility and components may also be supplied when required.

normal dependencies are recorded in the generated <project>Config.cmake. build and dev dependencies are intentionally absent from consumer package metadata.

This model does not integrate with Spack. A Spack development environment declares and resolves its own dependency set independently, including any package-name differences. provider = "cmake" describes how the CMake backend resolves this project dependency; it is not a package-manager identity shared with Spack.